The present invention relates to multiple value image input equipment which reads picture images having multiple chromatic gradation, for example, 64 chromatic grades or tonal ranges, and, more particularly, to such equipment which corrects inaccuracies in the expressions of such chromatic grades due to factors such as dispersion in the image sensor sensitivity.
Picture image processing with a computer system has become relatively widespread, as has the use of various picture image input equipment to read picture images, such as from photographs, for later processing. Most of the picture image input equipment currently in use converts input picture images into signals having binary values. This is typical of picture image input blocks in conventional facsimile machines. Even though that type of equipment can reproduce characters and line drawings, it cannot reproduce halftone images.
For halftone reproduction, recording processes such as the "Dither Process" have been developed. Along with the development of such processes, multiple value picture image input equipment, which is capable of representing scanned picture images in multiple value data, has become very important.
Conventional multiple value image input equipment applies shading correction to the picture image data obtained by scanning. This is needed because the levels of the signals output by the pixels are usually not in agreement even when reading the same part of an original sheet which should have the same optical density. The lack of agreement may be due to various factors such as a lack of uniform sensitivity of the individual pixels in the image sensor or a lack of uniformity in the quantity of light reflected from the original sheet.
To address this problem, some image input equipment is provided with a white plate or density-indicating block arranged in an area outside the reading area of the original sheet along the reading or main scanning direction of a linear image sensor. The sensor reads this white color plate before reading the original sheet. The equipment then sets the signal level so that the value of the signal read from the plate will agree with the value of the signal produced by each of the individual pixels when reading a white color. In this manner, conventional multiple value image input equipment attempts to adjust the perceived density of picture images.
Some multiple value image input equipment that processes picture images with 64 or 256 chromatic grades or tones experiences differences among the signal levels detected by the individual pixels. The differences most often occur at levels closest to the black color even if the reference point for the white color is adjusted to the same reference point. Because of this deviation, the individual pixels will not necessarily indicate the same value for identical optical density.
This point is illustrated in FIG. 1(a) through FIG. 1(d). FIG. 1(a) illustrates a white color or shading correction plate 11. White color plate 11 is read with a linear image sensor 12 shown in FIG. 1(b). Linear image sensor 12 is composed of n pixels labeled "1" to "n," each of which reads picture images pixel by pixel.
FIG. 1(c) represents an example of the dynamic ranges of four of the pixels, namely, the first, the third, the fifth, and the n-th pixels. In this example, the first pixel changes its signal level in 64 stages from "0" to "63" between its reading of the white color plate 11 and its reading of the darkest color. This pixel is in an ideal state and can be used for indicating the chromatic gradation at 64 levels, as shown in FIG. 1(d), without correction.
The third pixel, on the other hand, has a dynamic range from -"1" to "62." This pixel can be used for indicating chromatic gradation at the 64 levels from "0" to "63" by adding 1 to all the levels, as shown in FIG. 1(d), in order to adjust the "0" the signal level so that it is recorded when the white color plate 11 is read with this pixel. In other words, an additive correction is all that is required for the output from the third pixel in this example.
The fifth pixel, however, changes its signal level from "10" to "55" between its reading of the white color plate 11 and its reading of the darkest color. Therefore, even if a correction is made by subtracting "10" each from each of the levels, the chromatic gradation indicated by this pixel will only be from "0" to "45," as shown in FIG. 1(d). The result is that a black color is corrected to a gray color and a gray color is corrected to a brighter gray color. It is not possible to make a complete correction of the fifth pixel merely by adding.
The n-th pixel shows the opposite state. Its dynamic range is wider than the usual one, extending from "-5" to "67." Therefore, when "5" is added to each of the measured levels, the levels of the chromatic gradation will range from "0" to "72" the result is that the comparatively dark gray color will be corrected to black while the other gray shades will be corrected to darker gray shades. With this pixel as well, it is not possible to make a complete correction merely by adding an offset.
The preceding description assumes that the sensitivity of the individual pixels have uniform characteristics across the entire brightness spectrum. There are cases, however, in which the sensitivity of the individual pixels are not uniform for the individual brightness levels. For instance, some pixels have higher sensitivity to lower brightness levels on the chromatic gradation scale while other pixels have higher sensitivity at the higher brightness levels. This raises the possibility that the same gray color is sensed by different pixels to have different chromatic grades because the individual pixels with identical dynamic ranges respond differently to changes in brightness.
The preceding description has addressed the case in which corrections are made to compensate for the differences in the sensitivity of the individual pixels in a linear image sensor. The same applies for corrections made for the influences of other factors, such as the quantity of light.
FIG. 2 shows an example of the actual output levels from a typical multiple value image input device for which the density of the picture image data is represented as eight bit data, corresponding to 256 stages, and the output of the equipment is expressed in six bits, or 64 stages, with the solid line 14. The one-dot chain line 15 indicates ideal output characteristics. As FIG. 2 demonstrates, the output characteristics of a typical multiple value image input equipment are not simple because a variety of factors are at play. This situation renders it difficult to make proper corrections of chromatic gradation for the halftone reproduction.
In addition, fluctuations also occur in the output of each line in the direction of the subsidiary scanning with the linear image sensor. The causes of such fluctuations include the following factors:
(i) changes in the distance between the equipment and the sheet due to vibrations of the scanning unit which moves the linear image sensor in the subsidiary scanning direction, PA1 (ii) changes in the light quantity accumulating time due to fluctuations in the speed of the scanning unit also caused by the vibrations of the scanning unit, and PA1 (iii) changes in the quantity of light from the light source subsequent scanning operations.